Various types of current sensors are known.
In an open loop current sensor, a magnetic field generated by a sensed electrical current passing through a conductor is sensed with one or more magnetic field sensing elements. The open loop current sensor does not provide a feedback path such as that described below in conjunction with a closed loop current sensor.
In a closed loop current sensor, similarly, a primary magnetic field generated by a sensed electrical current passing through a primary conductor is sensed with one or more magnetic field sensing elements. However, in the closed loop current sensor, the one or more magnetic field sensing elements are also responsive to a secondary magnetic field generated by another electrical current passing through a secondary conductor disposed in a feedback arrangement. Thus, the one or more magnetic field sensing elements are responsive to a sum of the primary magnetic field and the secondary magnetic field. In the dosed loop current sensor, a magnitude of the secondary magnetic field is related to a magnitude of the primary magnetic field. In some embodiments, the secondary magnetic field and the primary magnetic field are equal in magnitude and opposite in sign, so that the stun of these fields, sensed by the one or more magnetic field sensing elements, is near zero.
Referring now to FIG. 1, a closed-loop current sensor 10 includes a magnetic sensing element 22, for example, a giant magnetoresistance (GMR) element. The magnetic field sensing element 22 is configured to generate a magnetic-field-responsive signal 22a. The closed-loop current sensor 10 also includes a primary conductor 12 configured to conduct a primary current 14, i.e., a sensed current 14, between two nodes 16, 18. The two nodes 16, 18 can, for example, correspond to pins of an integrated circuit.
In some embodiments, the primary conductor 12 comprises a coupling of two pins of an integrated circuit, for example, by way of a coupling of leads of a leadframe of the integrated circuit. This arrangement is described more fully below in conjunction with FIGS. 11 and 12.
The closed loop current sensor 10 also includes an amplifier 24 coupled to receive the magnetic-field-responsive signal 22a and configured to generate an amplified signal 24a. A transconductance amplifier 26 is coupled to receive the amplified signal 24a and configured to generate a current signal 26a. A secondary conductor 20 is coupled to receive the current signal 26a at one end, and is coupled to a node 28 at another end. The node 28 can, in some embodiments, correspond to another input of the integrated circuit.
A resistor 30 is also coupled to the node 28. Thus, in some embodiments, the resistor 30 is outside of the integrated circuit. A voltage signal 32 is generated by the current signal 26a passing through the resistor 30.
The closed-loop current sensor 10 is responsive to a primary magnetic field generated by the primary current 14, IP passing through the primary conductor 12, and also responsive to a secondary magnetic field generated by the current signal 26a, Ic, passing through the secondary conductor 20. The closed loop current sensor 10 uses negative feedback to generate the secondary current, IC, which, in turn, generates the secondary magnetic field, which can be oriented to oppose, at the magnetic field sensing element 22, the primary magnetic field generated by the primary current, IP. The secondary current 26a, IC, is proportional to the input (i.e., sensed) current 14, IP, as follows:
                                          I            C                    =                                                    k                P                                            k                C                                      ⁢                                          A                L                                            1                +                                  A                  L                                                      ⁢                          I              P                                      ,                            (        1        )            where:AL=gmkCkGA0,  (2)and where:
kP=primary conductor coupling factor;
kC=secondary conductor coupling factor;
AL=loop gain;
gm=transconductance of the transconductance amplifier 26;
kG=magnetic sensor gain; and
A0=low-frequency open loop gain of the amplifier 24
If the low frequency open loop gain, A0, is selected to be very large so that AL>>1, the sensitivity, SI=IC/IP, of the closed-loop current sensor depends only upon the coupling factors, kP and kC:
                                          S            I                    ≈                                    I              C                                      I              P                                      =                                            k              P                                      k              C                                .                                    (        3        )            
Typically, kP<<kC and the secondary current, IC, 26a is a small fraction of the primary current, IP, 14. Coupling factors kP and kC are generally insensitive to temperature, supply and package stress variations. Sensitivity is, therefore, stable and sensitivity drift is low. However, kC and especially kP will vary due to tolerances (e.g., positional tolerances) in IC fabrication and packaging. One-time factory sensitivity trim is often necessary to compensate for these variations.
The secondary current, IC, 26a can be routed through the sense resistor 30 to produce the magnetic field sensor output voltage signal 32. Voltage-output sensitivity is described by:
                                          S            V                    =                                                    v                0                                            I                P                                      =                                                            k                  P                                                  k                  C                                            ⁢                              R                S                                                    ,                            (        4        )            which indicates dependence upon a value, RS, of the resistor 30. Sensitivity can be trimmed to account for fabrication and packaging variations by adjusting a value of the resistor 30, but this cannot be properly done in the factory if an external resistor is used.
In order to maintain low sensitivity drift of the magnetic field sensor 10, it is desirable that the resistor 30 be a precision resistor external to the current sensor 10. Trimming can be performed by adding an internal programmable-gain amplifier that senses the voltage across the resistor 30, but this comes at the expense of an extra package pin, extra circuit area, and unwanted offset errors introduced by the extra circuitry.
The only remaining method for trimming sensitivity in a closed loop magnetic current sensor is to change one of the coupling factors, kP or kC. The coupling factor, kP, associated with the primary conductor 12 tends to be fixed by integrated circuit packaging structure and dimensions and cannot be easily altered. In particular, it is often desirable that the primary conductor 12 be relatively large so as to provide a relatively low resistance, thereby allowing a relatively high value of the primary (i.e., sensed) current 14. The coupling factor, kP, associated with the secondary conductor 20 tends to be fixed by integrated circuit fabrication and is also difficult to alter for trimming.
It would be desirable to provide a closed loop current sensor for which a sensitivity can be adjusted by way of adjusting a coupling factor.